Experimentally determining the 3-D structure of the G protein-coupled receptor (GPCR), CXCR4 and its complexes, the overall goal of the proposed study, is critical in developing a deeper understanding of its role in HIV viral entry and toxicity;availability of these data is expected to facilitate the development of new therapeutic strategies. Although CCR5 is the primary co-receptor responsible for HIV transmission, emergence of CXCR4-tropic (X4-tropic) viruses later in infection, correlates with a more rapid CD4 decline and a faster progression to AIDS. CXCR4 is also implicated in neuroaids, triggering neuronal cell death and possibly causing onset of dementia. Known conformational flexibility of GPCRs as well as results from preliminary studies have shown that stabilization of the CXCR4 receptor is a significant bottleneck in attempts to crystallize and solve its structures. The two PI's of this research proposal, one a chemist, and the other a crystallographer, propose to develop new chemical tools optimized for structural studies of the HIV co-receptor CXCR4, attempting crystallization and structural determination of the receptors and its complexes. Chemical tools will be designed and optimized for stabilizing, purifying, and detecting the receptor and for measuring its functional behavior. R21 studies will have two aims: (1) Develop tool compounds optimized for structural studies of CXCR4 and (2) Validate new ligands, detergents, and lipids by assessing their impact on protein sample production processes and by producing diffracting crystals of CXCR4. Results from R21 studies will be used to achieve the following in R33 phase of the work: (3) Determine the high-resolution X-ray structure of CXCR4, and (4) Assess the utility of new molecular tools and technologies in stabilizing and crystallizing CXCR4 in complex with gp120. Success of the R21 studies will be achieved by the production of diffracting CXCR4 crystals along with the production of new compound tools that significantly improves receptor sample behavior (e.g. improved stability over time, increased Tm). PUBLIC HEALTH RELEVANCE: Availability of a 3D structure of CXCR4 along with its complexes will lead to developing a deeper understanding of how the HIV virus enters the cell and will help in the design of new therapeutics. CXCR4 is also involved in important physiological process and have been implicated in a number of diseases such as cancer and availability of its structure will likewise have important biomedical implications.